The mainstream air bags were those bags made of nylon fabric coated with chloroprene rubber. As a substitute for the chloroprene rubber, air bags coated with silicone were recently developed for the purposes of improving heat resistance, weather resistance and flame retardance.
Air bag coating compositions based on chloroprene or silicone are generally prepared by adding curing agents, flame retardants, adhesion aids, reinforcements and the like to chloroprene rubber or silicone rubber, followed by diluting with organic solvents. The compositions are then applied to base fabrics of nylon or the like.
The air bag is of the design that it is normally folded to small dimensions, but instantaneously inflated when explosive gas is injected. On inflation, the coating applied to the air bag fabric is also instantaneously stretched following the air bag fabric. Thus the coating itself should be mechanically strong. Therefore, the currently available chloroprene and silicone air bag coating compositions both use base polymers of a relatively high molecular weight so that cured coatings thereof may have strength and elongation. The air bag coating compositions using high molecular weight base polymers, however, are quite difficult to apply to a coating thickness of 40 to 100 .mu.m normally required for air bags using knife coaters, offset roll coaters, gravure coaters or the like. The air bag coating compositions using high polymers must be diluted with organic solvents to a sufficient viscosity to allow easy coating.
However, the use of organic solvents has serious problems that they have the risk of ignition by static electricity especially in the working environment and that they are harmful to the operator by way of inhalation or skin contact. In addition, evaporated solvents impose the problems that their recovery is very expensive and if not recovered, they cause air pollution. For these reasons, coating compositions free of organic solvents are now desired in all industrial fields.
Among means for changing the air bag coating composition to a solventless system, the simplest way is to reduce the viscosity of the base polymer to a sufficient level to allow for coating by means of a knife coater or the like. However, a base polymer having a lower degree of polymerization is somewhat low in mechanical strength so that cracks can form on the coating surface upon air bag inflation and hot explosive gas can bleed out therethrough. If the viscosity is reduced too low, the composition will strike through a base fabric or plain weave fabric of nylon fibers, resulting in a less smooth surface. If the composition on the rear surface is cured as struck-through and then wound up, the coated fabric gives rise to blocking. Additionally, the struck-through composition will adhere to rolls of the coating machine, adversely affecting operating efficiency and outer appearance.
It was thus proposed to use aqueous silicone emulsion compositions which cure into silicone elastomers as the solventless air bag coating composition. Prior art aqueous silicone emulsion compositions did not firmly bond to air bag base fabrics or plain weave fabrics of polyimide or polyester fibers.
Japanese Patent Application Kokai (JP-A) No. 16553/1981 discloses a silicone emulsion composition comprising an anionically stabilized hydroxylated diorganopolysiloxane, colloidal silica, and an organic tin compound or organic amine compound at pH 9 to 11.5. Undesirably, this composition is limited in many aspects upon application because of its poor bond to certain base fabrics and strong alkalinity.
A variety of proposals were made in order to improve the adhesion of compositions of this type. For example, JP-A 131661/1979 discloses an organopolysiloxane latex composition obtained by emulsion polymerizing a cyclic organosiloxane and a functional group-containing organotrialkoxysilane (e.g., aminoalkyltrialkoxysilane) in the presence of a sulfonic acid or quaternary ammonium salt surfactant. USP No. 3,817,894 discloses a silicone latex composition comprising a siloxane block copolymer including dimethylsiloxane and monophenylsiloxane units, water, a cationic surfactant, a nonionic surfactant, a filler and an amino-functional alkoxysilane. These compositions, however, are less stable due to the presence of cationic surfactants.
In general, silicone latexes are often prepared using anionic emulsifiers rather than cationic emulsifiers from the standpoint of stability of silicone latexes (see Japanese Patent Publication No. 18800/1968, for example). These systems have the serious drawback that if silicone latexes stabilized with anionic emulsifiers are combined with amino-functional silanes or partial hydrolyzates thereof for improving adhesion, then substantial thickening (viscosity increase) or gelation (formation of insoluble matter and precipitates) occurs.